This invention may be used in the diffusion into a body of water of gas bubbles of any type, and has particular relevance to the diffusion into water of bubbles of air or of pure oxygen. In every case, the principal purpose for which the invention is used is to increase the rate at which a gas is absorbed by a body of water.
Aeration or oxygenation is very important in the operation and maintenance of water lagoons on fish farms or shrimp farms, or other ponds used for the culture of various other forms of aquatic life.
The level of dissolved oxygen can be said to be the most important water quality factor for fish. It is believed that in the operation of a fish farm, oxygen depletion probably results in more economic losses than all other factors combined. The maintenance of adequate oxygen levels is extremely important also in a culture pond for growing shrimp or other forms of aquatic life.
Aeration of a body of water has typically been effected by increasing the area of contact between the air and water through releasing air bubbles into the water below the surface of the water, through agitating the surface water, or both. Methods of agitating surface water include the use of turbines to propel water into the air, the use of paddle wheels to splash the surface of the water, flowing water by gravity over a rough surface, and others.
In the release of air bubbles of various sizes beneath the surface of the water, a variety of types of air diffusers have been employed. In all these devices, some of the air is absorbed as the bubbles rise through the water, and the air that is not absorbed escapes from the surface of the water and may or may not be captured for recirculation.
Prior workers in this field who have addressed themselves to the aeration of the water in a fish culture pond have long recognized the problem presented by the fact that the transit time of air bubbles moving up through the water from an air diffuser resting on the bottom of the container is quite short if the container is shallow. One result of the use of helical tube dividers in U.S. Pat. Nos. 1,144,342, 3,452,966 and 3,852,384 is to increase the gas-liquid contact time as the gas bubbles spiral upward.
Other prior art patents, such as U.S. Pat. Nos. 3,969,446, 4,272,461, and 4,336,144, take a different tack, for the express purpose of improving the absorption of gas in the water being treated. These latter patents do not attempt to increase the gas-liquid contact time, either by the expedient employed by applicant or by any other means but instead disclose measures to convert large "slugs" or "blurps" of air into fine gas bubbles.
One example of this latter type of device is an aerator made and sold commercially under the above mentioned U.S. Pat. No. 3,969,446, and described in a brochure entitled "Ramco Water Recovery Systems" published by Ramco Sales, Inc. of San Pedro, Calif. in May 1985 and revised in February 1987, and first advertised in the February 1985 issue of Aquaculture Magazine. The prior art universally teaches that it is not feasible to locate an aerator that releases air bubbles any deeper in the body of water being treated than necessary because such aerators operate much less efficiently in deep water. The particular prior art device just mentioned made a limited exception to this rule, and lowered the point of introduction of air bubbles by only the few inches below the bottom of the fish pond being treated that were required to avoid any obstruction to seining the pond to harvest the fish. However, so far as applicant is aware, prior to the present invention no one had lowered the point at which gas bubbles are introduced far enough below the bottom of the body of water being treated--while at any given time removing only a small fraction of the water to be aerated and returned to the body of water--to achieve the advantages of the system, method and apparatus of this invention.
It has long been recognized that it is desirable, with air lift pumps used as diffusers, to draw water from a point deep in a pond. At the same time, it has been universally believed that the air must be introduced at a much shallower depth in order to minimize power consumption. Application has surprisingly discovered that with the system and method of this invention the advantages of a deep draw of water can be obtained, and at the same time the air diffuser can be operated very efficiently, even though the bubbles are introduced at an unusually deep point below the surface of the body of water being treated, and in fact well below the bottom of the body of water.
The so-called "U-tube" method of aeration of a moving stream of water--or of a body of water that is otherwise confined, but flows over a dam or weir located at one end of a basin in which the water is held to produce the hydrostatic head that is required for the method--has been known for many years. (See Richard E. Speece, "U-Tube Stream Reaeration," Public Works, August 1969, page 111, footnotes 1-4.) In this method the gas/liquid contact time is increased by introducing air bubbles near the top of one leg of a U-shaped tube, located below the bottom of a body of water, through which the entire body of water flows, first in the downward direction, then in the upward direction. In some applications, as an alternative to relying on the pressure from a head of water, the pressure that is required at the top of the downflow channel of the U-tube in order to produce a downward flow velocity great enough to counter the buoyancy velocity of the bubbles being introduced into the water is provided by a pump or other mechanical means.
A very considerable amount of research and development effort has been addressed to the improvement of U-tube aeration, in addition to other aeration techniques, as applied to the aeration and circulation of ponds used for fish culture, to the aeration of streams, and to the treatment of wastewater. As one example, the U-tube concept appears to have been employed not only for the aeration of natural streams but also in water treatment plants since 1958 (see Speece, above, page 111, footnote 4), and in fish aquaculture as well. And, as pointed out in 1979 by Nick C. Parker of the U.S. Fish and Wildlife Service Southeastern Fish Cultural Laboratory, Marion, Ala. (at the 1979 proceedings of the Fish Farming Conference and Annual Convention of Catfish Farmers of Texas, Jan. 17-19, 1979, at the Texas Agricultural Experiment Station, Texas A & M University), several investigators--including among others the above mentioned Richard E. Speece--had by 1969, 1971 and 1973, respectively, developed various techniques and equipment to increase aeration and circulation in aquaculture units.
It is significant also that air lift pumps have been known for far more than a century, whether employed within a body of water to be aerated or merely to raise the water to a higher level, as in an artesian well. Gas diffusers of one sort or another have likewise been known for more than a century. However, so far as applicant is aware, no one prior to applicant has combined (1) an air diffuser that functions as an air lift pump with (2) the concept of an underground flow path (3) for a quantity of water that is only a small fraction of the body of water being treated, to produce the system and method of the present invention.
The introduction and flow of air bubbles through large channels extending far below the bottom of a body of liquid has been employed in the so-called "deep shaft" system for carrying out the activated sludge process for treating liquid sewage. (See Bailey et al. U.S. Pat. No. 4,351,730 issued in 1982, with a publication date of May 18, 1977 for the corresponding patent in Great Britain, No. 1,473,665.) However, this has been for a special purpose--to diminish the land area required for sewage works of a given capacity--and this fact has resulted in a very different system and method of treatment of the body of liquid. The objective of reducing the required land area is achieved in the deep shaft system by continuously circulating a single stream, which is as large as practicable of as much of the so-called "mixed liquor" (the liquid sewage containing the previously developed biological floc particles that are referred to as "activated sludge") as is feasible.
In the language of the broadest claims of U.S. Pat. No. 4,351,730, the objective of reducing the required land area is achieved by "continuously circulating the bulk of sewage contained in the basin-downcomer-riser system at any one time" deep below ground level in a system of very large channels. This continuous circulation is accomplished by keeping an enormous volume of mixed liquor below ground level at all times--preferably as much as about 2 to about 6 or more times the volume of the mixed liquor that is in the basin above ground.
This emphasis on the circulation at all times of as much mixed liquor as feasible is directly contrary to applicant's emphasis (explained below) on circulating at any given time only a small fraction of the water in the body of water being treated.
In addition to requiring that a very large portion of the mixed liquor must be below ground at all times, Bailey et al. do not teach the use of fine air bubbles or even of medium size air bubbles. On the contrary, they teach the use of spargers, which can only result in the introduction into the mixed liquor of very large "bubbles," "slugs" or "blurps" of air.
It is reported that this deep shaft approach to the activated sludge process of treating sewage has been employed in numerous projects around the world since the mid-1970's. However, so far as applicant is aware, neither that extended use, nor the issuance in 1982 of the U.S. patent just referred to, nor the earlier publication (in 1977) of the corresponding British application for patent, suggested the present invention to anyone, whether in the field of water treatment, sewage treatment or fish aquaculture.
The teaching in Bailey et al. of the use of large slugs of air agrees with the conclusion of other skilled workers in this field that in an air lift pump a single air injection hole 1.25 cms. in diameter was preferable to a number of smaller holes. (Nick C. Parker and Mary Anna Suttle, "Design of Airlift Pumps for Water Circulation and Aeration in Aquaculture," 6 Aquacultural Engineering (1987) 97, 106-107.) The long held belief that fine bubbles should not be used in an airlift pump is shown by the acceptance on the part of the two authors just cited of the conclusion reached 63 years earlier that "small fine bubbles provided no advantage because small bubbles quickly coalesced into larger bubbles as they traveled up through the water column" of an airlift pump. (C. N. Ward, "An experimental study of airlift pumps," Water Works (1924) pgs. 1275-1278.)